1. Technical Field of the Invention
The present invention relates to a die cushion apparatus for a press machine. In particular, the invention relates to a feature in the operating mechanism of a die cushion apparatus.
2. Description of the Related Art
Conventionally, with a single-acting press, when a cylindrical container, for instance, is pressed, the blank is prevented from being wrinkled at the periphery thereof. That is, the press is provided with a die, and a punch is arranged in the lower mold. The punch is fixed to the bolster. A blank holder is provided outside the punch to support the periphery of the blank. This blank holder is supported by cushion pins attached to a die cushion apparatus.
The lower structure of a conventional press machine is described referring to drawings. FIG. 1 is a sectional side view of the lower structure of a press machine incorporating a conventional die cushion apparatus.
The lower structure 1 of the press machine is provided with a die cushion apparatus 300, a blank holder 10, a die 20, a punch 30, a bolster 40, a press bed 50 and a slide 60.
The press bed 50 is the lower structure of the press frame, and is connected to the upper structure by means of an upright member, and supports the weight of the whole press. The bolster 40, the lower surface of which is resting on the press bed 50 is a base that supports the punch 30. The punch 30 is a lower die, the lower surface of which is supported by the bolster 40. The die 20 is an upper mold, the upper surface of which is attached to a slide 60. The slide 60 holds the die 20, is supported on the press frame in a manner such that it is free to move up and down, and is driven up and down by a drive mechanism. The blank holder 10 is a device that sandwiches the periphery of the blank between the upper surface 11 of the blank holder 10 and the lower surface 21 of the die 20 when the machine presses the blank between the die 20 and the punch 30. The blank holder 10 is also a device that supports the blank after the pressing process is finished and transfers it to an unloading device, and the lower surface of the holder is supported by the die cushion apparatus 300. The die cushion apparatus 300 is a device for supporting the blank holder 10, and is attached to the press bed 50.
Here, the functions that the die cushion apparatus 300 must provide are described. The primary function is the requirement to reduce the noise and vibration produced by the die 20 and the punch 30 during the pressing process (this is called the cushion function). In addition, another function of the apparatus is to clamp the outer periphery of the blank between the lower surface 21 of the die 20 and the upper surface 11 of the blank holder 20 to prevent the outer periphery of the blank from being creased when the die 20 presses the blank (this is called the crease-pressing function). Also, to protect the outer periphery of the blank from being damaged when the die 20 passes bottom dead center and starts to rise, the blank holder 10 that supports the blank is locked so that it does not travel past the bottom dead center position (this is called the locking function). Furthermore, this locking function is preferably also capable of lowering the blank holder 10 with the blank resting on it from the bottom dead center position by a predetermined distance (for example, about 3 mm). Moreover, when the die 20 passes bottom dead center and travels to the top dead center, the blank must be quickly transferred to an unloading device. For this purpose, another function is required that is to lift the blank holder 10 that supports the blank by a predetermined distance (for example, about 35 mm) and then stop the holder (this is called the secondary lifting function).
Next, the construction of a conventional die cushion apparatus is described. The die cushion apparatus is composed of pusher pins 310, a pusher pad 320, pneumatic cylinders 330, a hydraulic servo cylinder 340, a hydraulic servo valve 350, a cushion stroke sensor 360, a hydraulic unit 370 and a hydraulic servo controller 380, to provide the aforementioned functions.
The pusher pins 310 are rod-shaped structures that support the blank holder 10. The pusher pins 310 penetrate the bolster 40, support the lower surface of the blank holder 10 at the top end thereof, and are supported by the pusher pad 320 at the bottom end thereof.
The pusher pad 320 is a structural body that supports the pusher pins 310, and is disposed below the bolster 40 in a manner such that it is free to move in the up/down direction.
The pneumatic cylinders 330 are air-type RAM cylinders that support the pusher pad 320 from below, and are installed on the press bed 50. The cylinder members of the pneumatic cylinders 330 are fixed to the lower surface of the pusher pad 320, and the lower ends of the RAM piston members are supported by the press bed 50. The cylinder members engage with the RAM piston members in such a manner that they are free to move up and down. The pneumatic cylinders 330 are connected through air piping to an air source (not illustrated).
The hydraulic servo cylinder 340 is a dual-rod-type hydraulic servo cylinder which is attached to the press bed so that the rods of which can move freely in the up/down direction. The upper rod 341 is connected to the pusher pad 320.
The hydraulic servo valve 350 is a servo control valve for the hydraulic servo cylinder 340, that drives the upper rod 341 of the hydraulic servo cylinder 340 with a preferred stroke, operating force and speed under the control of the hydraulic servo controller 380.
The cushion stroke sensor 360 is a sensor for measuring the travel of the pusher pad 320, the output of which is transmitted to the hydraulic servo controller 380.
The hydraulic unit 370 is a hydraulic unit dedicated to the hydraulic servo cylinder 340, and supplies the hydraulic servo cylinder 340 with an operating fluid through the hydraulic servo valve 350.
The hydraulic servo controller 380 is a control device that actuates the hydraulic servo valve 350, and outputs control signals to the hydraulic servo valve 350 based on positional information sent from the cushion stroke sensor 360.
Next, the procedure by which the die cushion apparatus performs the required functions is described. FIG. 2 shows the movement of the die passing through points 2, 4, 3 and 5 and the movement of the blank holder passing through points 6, 7, 8 and 9. The movements of the lower surface of the die moving up and down and the upper surface of the blank holder moving up and down are shown with elapsed time on the abscissa.
The movement curve of the die is similar to that of a sine wave, although it may differ depending on the mechanism of the press machine. The top and bottom of the movement curve are called top dead center point 2 and bottom dead center point 3, respectively.
When the die is located at the top dead center point 2, the blank holder 10 remains stationary at an intermediate predetermined point 6 between the top dead center point 2 and the bottom dead center point 3.
The die 20 moves down from the top dead center point 2 along the movement curve 4, and reaches the bottom dead center point 3 while pressing the blank against the punch 30. The blank holder 10 is pushed down by the die 20 and moves to the bottom dead center point 3. Meanwhile, the outer periphery of the blank is clamped between the upper surface 11 of the blank holder 10 and the lower surface 21 of the die 20, and is pressed with a predetermined force produced by the pneumatic cylinders 330. The force prevents the outer periphery of the blank from being creased. Also, since the die cushion apparatus 1 presses the die 20 upwards with a predetermined clamping force created by the pneumatic cylinders 330, the noise and vibration that would otherwise be produced between the upper and lower molds during the pressing process is reduced.
When the die 20 passes the bottom dead center point 3 and moves along the rising curve 5, the hydraulic servo controller 380 detects information sent from the cushion stroke sensor 360 about the travel of the pusher pad 320, controls the hydraulic servo cylinder 340 via the hydraulic servo valve 350, and stops the pusher pad 320 by opposing the force from the pneumatic cylinders 330. In addition, the hydraulic servo cylinder 340 lowers the pusher pad 320 by a predetermined distance (for instance, about 3 mm). Consequently, the blank holder 10, with the blank, resting on it, is prevented from moving upwards at the bottom dead center point 3, and is moved further down from the bottom dead center point by a predetermined distance (for example, about 3 mm) to the lower position 8.
When the die rises from the bottom dead center point 3 towards the top dead center point 2, the hydraulic servo cylinder 340 raises the pusher pad 320 by a predetermined distance (for instance, about 35 mm) to position 9, and stops the pad thereof. The blank holder 10 on which the blank is resting stops at the position 9 at a predetermined elevation (for example, about 35 mm). An unloader receives the blank resting on the blank holder, and sends it to a subsequent process.
When the die 20 reaches the top dead center point 2, the hydraulic servo cylinder 340 lifts the pusher pad 320 to the initial standby position 6. The blank holder 10 remains at the intermediate predetermined position 6 between the top dead center point 2 and the bottom dead center point 3, and the condition has returned to the initial status of the cycle. Subsequently, this cycle is repeated and pressing work is carried out.
In the case of the aforementioned die cushion apparatus, because a hydraulic servo cylinder is used to control the position of the pusher pad, the apparatus has the advantage that the movement can be freely chosen to provide the preferred positions, however on the other hand, there are disadvantages caused by the use of the hydraulic servo cylinder.
First, the hydraulic servo system must use an operating fluid which is cleaner than that of conventional hydraulic devices. If the cleanliness of the oil becomes even slightly reduced, a servo-lock phenomenon seen only in hydraulic servo devices occurs, causing the hydraulic servo cylinder to stop. Therefore, the cleanliness of the operating fluid should be maintained at a specified high level, so controlling the cleanliness of the operating fluid is a considerable burden.
Secondly, since the hydraulic servo valve controls the hydraulic servo cylinder, there is a time delay in the response of the servo system. The hydraulic servo controller sends a control signal to the hydraulic servo valve at a predetermined timing taking the delay into consideration. Work to set the timing must be done very precisely, and sometimes, the position of the sensor must be readjusted. If the pressing speed is changed or the dies are changed, the control system must be readjusted.
Consequently, the die cushion apparatus using a conventional hydraulic servo system is expensive and is difficult to handle and maintain, which is a practical problem.
The present invention aims at solving the above-mentioned problems, and provides a die cushion apparatus that is less expensive and can be easily handled and maintained, compared to a conventional die cushion apparatus.
To achieve the object described above, the die cushion apparatus according to the present invention that can hold the periphery of a blank during the process of pressing the blank using dies, is provided with a support member that can hold the blank, gas pressure cylinders that push up the support member, a hydraulic cylinder of which the upper rod is connected to the support member, a pneumo-hydraulic converter for secondary lifting with a piston that partitions the interior of the converter into an oil chamber that communicates with the oil chamber of the hydraulic cylinder on the side of the aforementioned rod and a gas chamber, a check valve that allows oil to flow from the oil chamber on the side opposite to the above-mentioned rod to the oil chamber on the side of the aforementioned rod, and a drain port that communicates with the oil chamber on the side of the aforementioned rod of the hydraulic cylinder; when the die passes the bottom dead center point, the drain port is closed, and when the die is moving up from the bottom dead center point to the top dead center point, the pressure in the gas chamber of the pneumo-hydraulic converter for secondary lifting is reduced and the piston is driven to the gas chamber side.
According to the above-mentioned configuration of the present invention, the support member supports the blank from below, the gas pressure cylinders push the support member upwards, the upper rod of the hydraulic cylinder is connected to the support member, and the above-mentioned rod, support member and blank are pushed upwards as a single unit by the gas pressure cylinders.
The check valve prevents oil from flowing from the oil chamber on the aforementioned rod side to the oil chamber at the opposite end, closes the drain port communicating with the oil chamber on the above-mentioned rod side of the hydraulic cylinder, and can confine oil in the oil chamber on the aforementioned rod side of the hydraulic cylinder.
The pneumo-hydraulic converter for secondary lifting is provided with a piston that partitions the interior into an oil chamber communicating with the oil chamber on the above-mentioned rod side of the hydraulic cylinder and a gas chamber, oil from the oil chamber on the above-mentioned rod side of the hydraulic cylinder can be transferred into the oil chamber of the pneumo-hydraulic converter for secondary lifting, by moving the piston towards the gas chamber.
Oil in the oil chamber on the aforementioned rod side of the hydraulic cylinder can be confined by closing the drain port when the die passes the bottom dead center point.
The operating fluid in the oil chamber on the above-mentioned rod side of the hydraulic cylinder can be transferred into the oil chamber of the pneumo-hydraulic converter for secondary lifting by decreasing the pressure in the gas chamber of the pneumo-hydraulic converter for secondary lifting and allowing the piston to move towards the gas chamber side during the process of moving the die from bottom dead center to top dead center.
In addition, die cushion apparatus according to the present invention is provided with a pneumo-hydraulic converter for locking, with a piston that partitions the interior into an oil chamber communicating with the oil chamber on the aforementioned rod side of the hydraulic cylinder and a gas chamber; when the die passes the bottom dead center point, the pressure in the gas chamber of the pneumo-hydraulic converter for locking is increased and the piston is moved towards the oil chamber side.
According to the above-mentioned configuration of the present invention, the pneumo-hydraulic converter for locking is provided with a piston that partitions the interior into the oil chamber communicating with the oil chamber on the above-mentioned rod side of the hydraulic cylinder and the gas chamber; by moving the piston towards the oil chamber side, the operating fluid can be transferred into the oil chamber on the aforementioned rod side of the hydraulic cylinder while the die is passing through the bottom dead center point. As the piston is moved towards the oil chamber side by increasing the pressure in the gas chamber of the pneumo-hydraulic converter for locking, the operating fluid in the oil chamber of the locking pneumo-hydraulic converter can be transferred into the oil chamber on the above-mentioned rod side of the hydraulic cylinder.
In the die cushion apparatus according to the present invention, the above-mentioned drain port is a hole that penetrates the wall of the oil chamber of the pneumo-hydraulic converter for locking, the piston of the pneumo-hydraulic converter for locking closes the drain port when it is moved to the oil chamber side, and the piston of the pneumo-hydraulic converter for locking opens the drain port, when it is moved to the gas chamber side.
Using the aforementioned configuration of the present invention, oil in the oil chamber of the above-mentioned rod side of the hydraulic cylinder can be drained through a hole that penetrates the wall of the oil chamber in the pneumo-hydraulic converter for locking. When the piston of the pneumo-hydraulic converter for locking is moved to the oil chamber side, the piston of the pneumo-hydraulic converter for locking closes the aforementioned drain port. Once the piston of the pneumo-hydraulic converter for locking is moved to the gas chamber side, the piston of the pneumo-hydraulic converter for locking opens the drain port.
In addition, the die cushion apparatus according to the present invention is devised such that the pneumo-hydraulic converter for locking is located in the piston of the pneumo-hydraulic converter for secondary lifting.
According to the aforementioned configuration of the present invention, the pneumo-hydraulic converter for locking is installed in the piston of the pneumo-hydraulic converter for secondary lifting, and the pneumo-hydraulic converter for locking can be integrated into a single body with the pneumo-hydraulic converter for secondary lifting.
Moreover, the die cushion apparatus of the present invention is configured in such a manner that the aforementioned drain port always communicates with the hole that penetrates the wall of the pneumo-hydraulic converter for secondary lifting.
By virtue of the above-mentioned configuration of the present invention, the aforementioned drain port can always communicate with the hole penetrating the wall of the pneumo-hydraulic converter for secondary lifting, and oil in the oil chamber on the above-mentioned rod side of the hydraulic cylinder can be drained through the hole penetrating the wall of the oil chamber in the pneumo-hydraulic converter for locking and the hole that penetrates the wall of the pneumo-hydraulic converter for secondary lifting.
Furthermore, the die cushion apparatus according to the present invention is composed such that the pneumo-hydraulic converter for secondary lifting is built into the cylinder of the pneumo-hydraulic converter for locking. The above-mentioned configuration of the present invention enables the pneumo-hydraulic converter for secondary lifting to be installed in the cylinder of the pneumo-hydraulic converter for locking and the pneumo-converters for secondary lifting and locking can be integrated into a single body.
In addition, the die cushion apparatus based on the present invention incorporates pneumo-hydraulic converters consisting of pneumo-hydraulic-based intensifiers.
In the configuration mentioned above according to the present invention, the pneumo-hydraulic converters can be driven by a low-pressure gas because the pneumo-hydraulic converters are pneumo-hydraulic-based intensifiers.
Other objects and advantages of the present invention can be revealed by the following descriptions referring to the attached drawings.